Method for connecting shape-memory material and steel or copper material

ABSTRACT

The process is to be used to connect a component made from a shape memory alloy to a thin component made from steel material or a copper material. The intention is to provide a spot-welding technique using electrodes made from a Cu alloy. The intention is to establish a current intensity of over 1000 A, preferably over 2000 A, at an I 2 t value of over 5000 A 2 s, preferably over 8000 A 2 s, and a pressure of at least 30 MPa, preferably 80 MPa.

[0001] This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/DE01/02154 which has an International filing date of Jun. 8, 2001, which designated the United States of America, the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The invention generally relates to a process for producing a mechanical and electrical connection between a component made from a shape memory alloy and a component made from a steel material or a copper material.

BACKGROUND OF THE INVENTION

[0003] Various alloys with shape memory properties, which change shape when heated and return to their original form after the heating operation has ended, are known. For example, Ti—Ni and Ti—Ni—Cu alloys of various compositions, which are shortened by several percent when they are heated, for example, to 200° C., are disclosed in “Materials Science and Engineering”, Vol. A 202, 1995, pages 148 to 156. Corresponding components are used in particular in circuit breakers (cf. for example the book “Engineering Aspects of Shape Memory Alloys”, published by Butterworth-Heinemann, London (GB) 1990, pages 330 to 337).

[0004] In line circuit breakers of this type, one problem that arises is that components made from shape memory alloys of this type are to be connected to components made from steel sheet or copper-plated steel sheet or to copper strips. It is necessary to ensure a high mechanical load-bearing capacity combined, at the same time, with good electrical conductivity and a low contact resistance. However, it has been found that connections of this type cannot readily be achieved by use of conventional welding technology. Therefore, known devices with corresponding connections have complex screw connections, clamp connections or pinched connections (for example by crimping).

SUMMARY OF THE INVENTION

[0005] Therefore, it is an object of an embodiment of the present invention to describe a process for producing a connection between components of this type which can withstand high mechanical and electrical loads.

[0006] Each of these three parameters may deviate from the value listed by up to ±10% without the connection properties deteriorating significantly.

[0007] Electrode materials made from a Cu alloy which can be used are to be found in DIN ISO 5182. Forms of electrode which are suitable for this purpose are known from DIN ISO 5821. It is particularly advantageous for the electrode material provided to be a Cu—X alloy which, in addition to Cu as its main constituent (≧50 atomic %), also contains, as the X component, at least one of the elements Ag, Be, Co, Cr, Cd, Fe, Hf, Mn, Mo, Nb, Pd, Pt, Ta, Ti, V, W, Zn, Zr. It is preferable to provide a CuCoBe alloy of the type Cu_(x)Co_(y)Be_(z), where 50≦x≦99.8, 0.1≦y≦20 and 0.1≦z≦20 (in each case in atomic %). In this case, it is the case that x+y+z≈100 (in atomic %) with the inclusion of small amounts of impurities, in each case amounting to less than 1 atomic %. The electrode materials which are to be used ensure good electrical conductivity, a high mechanical strength and good compatibility with the materials which are to be welded.

[0008] These electrode materials allow components made from said NiTi or NiTiCu shape memory alloy to be welded particularly successfully to copper-plated steel sheets. Corresponding connections with good electrical conductivity and a high mechanical load-bearing capacity are required in particular for circuit breakers with switching elements made from shape memory alloys (cf. the cited passage from the book “Engineering Aspects of Shape Memory Alloys”). Similar connection problems also arise if parts made from a shape memory alloy are to be joined to a metal sheet or a stranded conductor made from Cu or a Cu alloy. In this case too, the process according to an embodiment of the invention can be used particularly advantageously.

[0009] Naturally, the process according to an embodiment of the invention can also be used to spot-weld components made from other known shape memory alloys to metal sheets or stranded conductors made from steel material or copper material. For example, various suitable Ti₅₀Ni_(50-x)Pd_(x) shape memory alloys are described in “Intermetallics”, Vol. 3, 1995, pages 35 to 46 and “Scripta METALLURGICA et MATERIALIA”, Vol. 27, 1992, pages 1097 to 1102. Of course, it is also possible for other shape memory alloys to be used instead of the Ti—Ni alloys. By way of example, Cu—Al shaped memory alloys are suitable. A corresponding Cu—Zn24A113 alloy is described in “Z. Metallkde.”, Vol. 79, section 10, 1988, pages 678 to 683. A further Cu—Al—Ni shape memory alloy is described in “Scripta Materialia”, Vol. 34, No. 2, 1996, pages 255 to 260. Naturally, further alloying partners, such as for example Hf, may also be alloyed with the abovementioned binary or ternary alloys in a manner which is known per se.

[0010] According to an embodiment of the invention, this object can be achieved by the fact that a spot-welding technique is provided. Preferably this is done by using electrodes made from a Cu alloy, a current intensity of over 1000 A at an I²t value of over 5000 A²s and a pressure of at least 30 MPa being established. Spot welding as a special process of resistance pressure welding takes place in a known way (cf. for example the book “Schweiβtechnik” [welding technology] by H. J. Fahrenwaldt, pub. F. Vieweg & Sohn, Braunschweig 1992, pages 79 to 86).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] An embodiment of the invention can be based on the discovery that only when the parameters listed above are combined and the special electrodes are used, is a spot-welding technique between components made from the materials possible. Only then can the demands imposed be satisfied. Since the abovementioned electrode alloy is to be a base alloy, this indicates that the alloy may comprise only two components, as well as the usual impurities in each case amounting to at most 1% by weight, or smaller amounts, of less than 10 atomic %, of further elements may be added to these alloys. Consequently, the advantages achieved by an embodiment of the invention include a connection between the components which can withstand high mechanical loads and also satisfies the demand relating to good electrical conductivity is created for the first time. The component made from steel material or copper materials which is to be connected to the shape memory alloy part should generally, for thermal conductivity reasons or on account of the processing technique used, for example the possibility of forming a coil, be relatively thin, i.e. should have a maximum thickness in the connection area of in particular at most 2 mm.

[0012] The process parameters in each case merely represent a lower limit. For example, it is to be considered advantageous if a current intensity of over 2000 A is established, since it has been found that with higher current intensities it is possible to further improve the connection properties.

[0013] This also applies in particular if an I²t value of over 8000 A²s is provided.

[0014] Since the pressure also affects the quality of the connection, it is advantageous to set a pressure of at least 40 MPa, preferably of at least 80 MPa.

[0015] It is to be considered particularly advantageous if a combination of the following parameters is provided:

[0016] Current intensity: approx 2000 A,

[0017] I²t value: approximately 10000 A²s,

[0018] Pressure: approximately 150 MPa.

[0019] Conventional spot-welding devices can be used for the process according to an embodiment of the invention. Corresponding devices are to be found, for example, in the cited excerpt from the book “Schweiβtechnik”.

[0020] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

1. A process for producing a mechanical and electrical connection between a component made from a shape memory alloy and a thin component made from an optionally coated steel material or a copper material, characterized in that a spot-welding technique is provided using electrodes made from a Cu alloy, a current intensity of over 1000 A at an I²t value of over 5000 A²s and a pressure of at least 30 MPa being established.
 2. The process as claimed in claim 1, characterized in that a current intensity of over 2000 A is established.
 3. The process as claimed in claim 1 or 2, characterized in that an I²t value of over 8000 A²s is established.
 4. The process as claimed in one of the preceding claims, characterized in that a pressure of at least 40 MPa, preferably of at least 80 MPa, is established.
 5. The process as claimed in claim 1, characterized in that a current intensity of approximately 2000 A, an I²t value of approximately 10000 A²s and a pressure of approximately 150 MPa are established, said values each being maintained to an accuracy of ±10%.
 6. The process as claimed in one of the preceding claims, characterized in that the electrode material provided is a Cu alloy which, in addition to Cu as its main constituent, also contains at least one of the elements Ag, Be, Co, Cr, Cd, Fe, Hf, Mn, Mo, Nb, Pd, Pt, Ta, Ti, V, W, Zn, Zr.
 7. The process as claimed in claim 6, characterized in that the electrode material provided is a CuCoBe alloy.
 8. The process as claimed in claim 7, characterized in that the electrode material provided is a Cu_(x)Co_(y)Be_(z) alloy, where 50≦x≦99.8, 0.1≦y≦20 and 0.1≦z≦20, with x+y+z≈100 (in each case in atomic %) with the inclusion of small amounts of impurities, in each case amounting to less than 1 atomic %.
 9. The process as claimed in one of the preceding claims, characterized in that a component made from the steel material or the copper material with a thickness of between 0.1 and 5 mm, preferably 0.5 and 2 mm, is provided.
 10. The process as claimed in one of the preceding claims, characterized in that a component made from a copper-plated steel sheet is provided.
 11. The process as claimed in one of claims 1 to 9, characterized in that a component made from a stranded copper conductor made from copper or a copper alloy is provided.
 12. The process as claimed in one of the preceding claims, characterized in that the shape memory alloy provided is a TiNi or TiNiCu or TiNiPd or CuAl or CuAlNi or CuAlZn alloy which, if appropriate, also contains further components, in each case in an amount of less than 5 atomic %.
 13. The process as claimed in claim 12, characterized in that a shape memory alloy to which Hf is alloyed is provided. 